I want to use something like the "in" operator available in other programming languages. I read many posts about that already. But, nothing that fits my needs.
What I wanted to do is a little bit different. Please see the following first example:
#include <iostream>
#include <initializer_list>
#include <algorithm>
bool operator ==(const int lhs, std::initializer_list<int>& il) {
return std::find(il.begin(), il.end(), lhs) != il.end();
}
int main() {
std::initializer_list<int> il{1,2,3,4,5};
std::cout << (3 == il) << '\n';
// std::cout << (3 == {1,2,3,4,5}) << '\n'; // Does not compile
}
But this does not compile. Presumably, because an initalizer list is no expression. Although there are exceptions. A std::initializer_list
maybe a function parameter, although also here expressions are expected.
And since any operator is basically also a function, my hope was, that I also can use std::initalizer_list
as argument.
But I cannot.
I tried the same approach by defining an own operator
with a name by misusing overloading of 2 operators. See below:
#include <iostream>
#include <vector>
// New operator: is_in
enum { is_in };
int operator < (const int& lhs, decltype(is_in)) { return lhs; }
int operator > (int lhs, std::vector<int>& rhs) { return std::find(rhs.begin(), rhs.end(), lhs) != rhs.end();}
int operator > (int lhs, std::initializer_list<int>& rhs) { return std::find(rhs.begin(), rhs.end(), lhs) != rhs.end(); }
int main() {
std::vector validValues{ 1, 2, 3, 4, 5 };
bool b = (5 <is_in> validValues);
// bool b = (5 <is_in> { 1, 2, 3, 4, 5 }); // Does not compile
std::cout << b << '\n';
}
Same principle, same problem . . .
Is there any way to make this happen?
CodePudding user response:
Brace init lists are only allowed in certain contexts by exceptions to the grammar. And while an overloaded operator involves a function call, it must still obey the standard grammar rules unless you invoke it by name (operator@
) in a function call.
So your named operator can work, but you must overload one of the operators for which an exception exists1. A rough summary of those overloadable operators is:
- Assignment, including compound assignment (
operator=
,operator =
). - Index (
operator[]
). - Function call (
operator()
).
None of those may be overloaded as a non-member, so getting symmetry (<is_in>
or =is_in=
), is not possible. But if forego that, we can do something like
#include <iostream>
#include <initializer_list>
inline constexpr class op {
template<typename T>
friend auto operator< (T const& t, op) {
struct {
T const & t;
bool operator=(std::initializer_list<T> il) {
return std::find(il.begin(), il.end(), t) != il.end();
}
} ret{t};
return ret;
}
} is_in;
int main() {
bool b = (5 <is_in= { 1, 2, 3, 4, 5 });
std::cout << b << '\n';
}
Which is not too pretty... I guess we can choose another operator instead of operator<
to perhaps improve the look of this, but it seems like folly in general. This is already a bit too obscure.
1 - On that front, you should know that overloading operators whose parameters are purely standard types is ill-advised. The standard can change the library definition at any time and break your code gloriously.
CodePudding user response:
You need to take the initializer_list
by const&
:
bool operator==(const int lhs, const std::initializer_list<int>& il)
std::cout << (3 == std::initializer_list{1,2,3,4,5}) << '\n';
For the is_in
test you could overload the comma operator and do something like this:
template<class T>
struct is_in {
is_in(const std::initializer_list<T>& il) : ref(il) {}
const std::initializer_list<T>& ref;
};
template<class T>
bool operator,(const T& lhs, const is_in<T>& rhs) {
return std::find(rhs.ref.begin(), rhs.ref.end(), lhs) != rhs.ref.end();
}
int main() {
bool b = (5, is_in{ 1, 2, 3, 4, 5 });
std::cout << b << '\n';
}